In typical catalytic cracking techniques, the fluid catalytic cracking unit (FCC) cracks petroleum-derived hydrocarbons using a catalyst to achieve gasoline production. Although efforts are made to reduce side effects from the reaction, a small amount of unwanted products are produced, which include: liquefied petroleum gas (LPG), cracked gas oil and the like, and coke, which is deposited on the catalyst and thereby reduces the catalyst's effectiveness. The spent catalyst is regenerated by burning away the deposited coke using air and heat before the catalyst is recycled back into the process.
However, in recent years, there has been a shift towards using FCC units as a means for producing light olefins, such as propylene, rather than for primarily producing gasoline. Utilizing an FCC unit in this manner can be economically advantageous, particularly when the oil refinery is highly integrated with other steps throughout the oil production process.
Earlier methods for producing light-fraction olefins by an FCC unit using heavy-fraction oils included contacting feed oil with a catalyst for a short time (U.S. Pat. Nos. 4,419,221; 3,074,878; and 5,462,652; and European Patent No. EP 315,179A), carrying out the cracking at high temperatures (U.S. Pat. No. 4,980,053), and using pentasil-type zeolites (U.S. Pat. No. 5,326,465 and Japanese Patent National Publication (Kohyo) No. Hei JP 7-506389).
However, the methods taught by the above references failed to produce sufficient light-fraction olefins selectively. For example, the methods taught by using a reduced catalyst contact time resulted in a decrease in the conversion of light-fraction olefins to light-fraction paraffins due to the methods' inhibition of a hydrogen transfer reaction. Furthermore, the lack of hydrogen transfer also led to a decrease in the conversion of heavy-fraction oils to light-fraction oils. The method teaching the use of the high temperature cracking reaction resulted in a concurrent thermal cracking of heavy-fraction oils, which thereby increased the yield of low-value, dry gases. Lastly, the use of pentasil-type zeolites enhanced the yield of light-fraction hydrocarbons by excessively cracking the gasoline. Therefore, there was still a need to produce a light-fraction olefin without causing unwanted side effects.
U.S. Pat. No. 6,656,346 ('346) discloses an improved process for the fluid catalytic cracking of a heavy-fraction hydrocarbon to produce a high yield of light-fraction olefins, while simultaneously producing a diminished amount of unwanted dry gases. The process of '346 achieves its objective by contacting the heavy-fraction oil with a catalyst mixture that consists of a specific base cracking catalyst and an additive containing a shape-selective zeolite at a high temperature. Furthermore, '346 discloses that the catalyst mixture preferably contains between 60-95 wt % of the base cracking catalyst, with the additive making up the remainder. Additionally, the base cracking catalyst contains an ultra stable Y-type zeolite that has less than 0.5 wt % of rare-earth metal oxide.
Moreover, '346 teaches that in the reaction zone, the fluid catalytic cracking may be affected within a fluid bed, in which the catalyst particles are fluidized with the heavy-fraction oil, or, may be effected by employing so-called riser cracking, in which both the catalyst particles and the heavy-fraction oil ascend through a pipe, or, so-called down flow cracking in which both the catalyst particles and the heavy-fraction oil descend through a pipe. '346 goes on to teach down-flow type reaction zones are preferable over up-flow reaction zones in order to reduce the deleterious effects of back-mixing that occurs in up-flow reaction zones.
In spite of this breakthrough, the method taught by '346 has some disadvantages. Most glaringly is the difficulty in managing the multitude of variables that must be observed and manipulated throughout the production cycle. Since the crude oil feed varies in composition, it can be extremely challenging for operations personnel to manually test the properties of the incoming stream and adjust the necessary variables accordingly. Furthermore, because the process taught by the prior art is complicated and contains a variety of manipulatable variables, it is virtually impossible for an operator to manually control the process, even with remote access via a computer, and achieve an optimum yield of light olefins. Additionally, typical numerical methods and statistical analysis do not provide an acceptable level of process control. Consequently, the methods taught by the prior art do not teach a method for carrying out the process in an efficient manner and ensuring that the yield of light-fraction olefins has been maximized. Furthermore, no methods teach optimizing the production of light-fraction olefins in relation to energy usage.